Fuel cells for combining hydrogen and oxygen to produce electricity are well known. A known class of fuel cells includes a solid-oxide electrolyte layer through which oxygen anions migrate; such fuel cells are referred to in the art as “solid-oxide” fuel cells (SOFCs).
In some applications, for example, as an auxiliary power unit (APU) for a transportation application, an SOFC is preferably fueled by “reformate” gas, which is the effluent from a catalytic liquid or gaseous hydrocarbon oxidizing reformer. Reformate typically includes amounts of carbon monoxide (CO) as fuel in addition to molecular hydrogen. The reforming operation and the fuel cell operation may be considered as first and second oxidative steps of the hydrocarbon fuel, resulting ultimately in water and carbon dioxide. Both reactions are preferably carried out at relatively high temperatures, for example, in the range of 700° C. to 1000° C.
A complete fuel cell stack assembly includes fuel cell subassemblies and a plurality of components known in the art as interconnects, which electrically connect the individual fuel cell subassemblies, in series. It is desirable that the chemical resistance of the interconnects be high, and accordingly some nickel-based metal alloys could be useful in this regard. However, it is also very desirable that the CTE of each interconnect matches the CTE of adjacent elements to which the interconnect is bonded during assembly, so that leaks will not develop when the assembly is heated to its operating temperatures. In general, the desirable corrosion-resistant alloys are not good CTE matches for sealing into a fuel cell stack, which has resulted in the use of ferritic stainless steels. However, ferritic stainless steels oxidize relatively rapidly at temperatures above 650° C.
What is needed is an interconnect that enjoys both the chemical resistance of nickel-based alloys and the thermal dimensional performance of ferritic stainless steels.
It is a principal object of the present invention to provide an interconnect periphery having a CTE approximating that of adjacent elements in a solid-oxide fuel cell stack.
It is a further object of the invention to provide long working lifetime of an interconnect in a solid-oxide fuel cell stack.